Consequences of divergent temperature optima in a host-parasite system

It was suggested that parasite infections become more severe with rising temperature, as expected during global warming. In ectothermic systems, the growth of a parasite and therefore its reproductive capacity is expected to increase with temperature. However, the outcome of the interaction depends on the temperature optima of both host and parasite. Here we used experimental infections of three-spined stickleback fish (Gasterosteus aculeatus) with its specific tapeworm parasite Schistocephalus solidus to investigate in detail the temperature optima for both host and parasite. We analyzed the fitness consequences thereof, focusing on growth and immunity of the host, and growth and offspring production of the parasite as fitness correlates. We checked for potential differences among populations, using the offspring of hosts and parasites derived from four study sites in Iceland, Germany and Spain that differ in average annual temperature ranging between 4.8 °C and 18.4 °C. We found differences in temperature optima of host and parasites that were quite consistent across the populations: while sticklebacks grew faster and had higher immune activity at low temperatures, the parasites did not even grow fast enough to reach sexual maturity in these conditions. By contrast, with increasing temperatures, parasite growth, egg production and offspring hatching increased strongly while host immunity and growth were impaired. Our results show that divergent temperature optima of hosts and parasites can have drastic fitness consequences and support the expectation that some parasites will benefit from global warming.

已有研究表明，在全球变暖的背景下，气温升高会导致寄生虫感染病情加重，这与学界的普遍预期一致。在变温生物系统（ectothermic systems）中，寄生虫的生长及其繁殖能力会随温度升高而增强，这是学界的常规预期。然而，宿主与寄生虫之间的互作结局，取决于二者各自的最适温度（temperature optima）。本研究以三刺鱼（three-spined stickleback，Gasterosteus aculeatus）及其特异性绦虫（tapeworm，Schistocephalus solidus）的实验感染体系为研究模型，对宿主与寄生虫各自的最适温度展开了详细探究。我们以此为基础分析了二者的适合度（fitness）后果，重点关注宿主的生长与免疫能力，以及寄生虫的生长、子代产出这两项适合度相关指标。我们选取了来自冰岛、德国与西班牙4个年均温介于4.8℃至18.4℃的研究地点的宿主与寄生虫子代，以此检验不同种群间可能存在的差异。研究发现，宿主与寄生虫的最适温度存在显著差异，且这一差异在所有研究种群中均保持一致：在低温环境下，三刺鱼生长速度更快、免疫活性更高，但此时寄生虫的生长速率甚至不足以达到性成熟阶段。与之相反，随着温度升高，寄生虫的生长、虫卵生成与子代孵化率均显著提升，但宿主的免疫能力与生长速度却受到了抑制。本研究结果表明，宿主与寄生虫之间最适温度的分化会对二者适合度产生剧烈影响，同时也印证了"部分寄生虫将从全球变暖中获益"这一学界预期。